1. Field of the Invention
The present invention relates to a flow rate sensor for outputting a signal in response to a flow rate of a fluid being measured, and relates to a flow rate sensor suitable for measuring an intake air flow rate of an internal combustion engine in an automobile, for example.
2. Description of the Related Art
Generally, in an automotive engine, etc., an air-fuel mixture including intake air and fuel is burnt in a combustion chamber in an engine body, and rotational output from the engine is extracted from the resulting combustion pressure, requiring that an intake air flow rate be detected in order to calculate an injection rate, etc., for the fuel with high precision.
Conventional flow rate measuring apparatuses used in such applications are constituted by a casing, a mount plate, a circuit board, a flow rate detecting element, etc. The casing is constituted by: a collar-shaped mount portion formed on a base end portion of the casing; a circuit accommodating portion formed into a generally rectangular overall box shape and disposed so as to extend from the mount portion inside a conduit; and a connector portion formed on the mount portion so as to be positioned outside the conduit, the connector portion sending and receiving signals to and from an external portion. Disposed in the circuit accommodating portion are: a circuit board mount recess portion disposed inside the conduit, the circuit board mount recess portion being surrounded by a peripheral wall forming a rectangular shape; and a mount plate interfitting groove formed by cutting away a portion of the peripheral wall of the circuit board mount recess portion positioned near a tip end of the casing. The mount plate is composed of: a circuit board mount portion mounted to the circuit board mount recess portion and formed by bending edge portions of the mount plate on the left and right at positions facing the circuit board mount recess portion; and an element mount portion formed integrally near a tip end of the circuit board mount portion, the element mount portion fitting into the mount plate interfitting groove such that a tip end extends outside the casing, a rectangular element accommodating recess portion for accommodating the flow rate detecting element being formed in the element mount portion. The circuit board is disposed on the circuit board mount portion, terminals of the circuit board and the connector portion being connected by a plurality of bonding wires. The flow rate detecting element is mounted to the element accommodating recess portion such that a portion of the flow rate detecting element positioned near a tip end projects outside the casing. The connection portions of the flow rate detecting element and the circuit board are connected by a plurality of bonding wires. Finally, the circuit board mount recess portion is charged with a sealant composed of a silicone gel so as to cover the circuit board from a front surface side. In this conventional flow rate measuring apparatus, the flow rate detecting element is passed through an element insertion aperture and disposed partway along a bypass passage by mounting the base end portion of the casing to a mounting aperture on the conduit. (See Patent Literature 1, for example.)
Patent Literature 1: Japanese Patent Laid-Open No. 2000-2572 (Gazette, paragraphs 0024 to 0029, paragraph 0034, FIGS. 1 to 4)
If a conventional flow rate measuring apparatus is used as an intake air flow rate detecting apparatus in an internal combustion engine, for example, it is normally plugged in immediately downstream from an intake air filtration apparatus. This intake air filtration apparatus is fastened to a vehicle body or chassis inside an engine compartment. Thus, vibrational acceleration was comparatively small since vibrations to which the flow rate measuring apparatus is subjected were transmitted through the vehicle body or the chassis.
However, in recent years, with reductions in the size of engine compartments, intake air filtration apparatuses are increasingly being installed immediately above the engine and fastened to the engine. The flow rate measuring apparatus may also be fastened to a throttle body. This throttle body is fastened directly onto the engine. The vibrational acceleration of the vibrations to which the flow rate measuring apparatus is subjected in such cases is extremely large compared to cases where the flow rate measuring apparatus is plugged in immediately downstream from the intake air filtration device fastened to the vehicle body or chassis.
In this conventional flow rate measuring apparatus, because the collar-shaped mount portion constituting the base end portion of the casing is mounted to the mounting aperture of the conduit, when vibration such as that described above occurs, the vibrational mode is one of cantilever support in which the mount portion constitutes a fixed end and the end projecting inside the conduit constitutes a free end. In such cases, the amplitude of the casing is greatest at the free end, the vibrations of the casing being propagated directly to the flow rate detecting element and the bonding wires by the mount plate mounted to the circuit accommodating portion of the casing. The vibration of the casing is further amplified if the casing has a resonance point relative to the oscillation frequency region generated by the internal combustion engine. Thus, there has been a risk that excessive stresses will be repeatedly applied to the connection portions between the bonding wires disposed inside the sealant and the flow rate detecting element and the connection portions between the bonding wires and the circuit board, inducing delamination or wire breakages at the connection portions between the flow rate detecting element and the bonding wires or between the circuit board and the bonding wires, bringing about abnormalities in the output from the flow rate measuring apparatus.
Furthermore, it is possible that the flow rate measuring apparatus may be subjected to mechanical shock from an assembly tool as the flow rate measuring apparatus is being mounted to the internal combustion engine, or that the flow rate measuring apparatus may be subjected to mechanical shock due to dropping of the flow rate measuring apparatus. Because the mount plate is mounted to the casing such that the tip end portion of the element mount portion extends outside the casing, there has been a risk that if the tip end portion of the casing is subjected to mechanical shock from an assembly tool or mechanical shock due to dropping of the flow rate measuring apparatus, vibration will propagate directly to the flow rate detecting element and the bonding wires, inducing damage to the flow rate detecting element or delamination or wire breakages at the connection portions between the flow rate detecting element and the bonding wires or between the circuit board and the bonding wires, bringing about abnormalities in the output from the flow rate measuring apparatus.
In order to solve such problems, it is also conceivable for the flow rate detecting element to be disposed outside the fixed end of the vibrational mode, in other words, outside the conduit, but because it is necessary to dispose the flow rate detecting element at a predetermined position inside the bypass passage, the electrical connection portions of the flow rate detecting element must inevitably be positioned in the vicinity of a central axial position in the conduit. In other words, there is no choice but to dispose the electrical connection portions of the flow rate detecting element in the vicinity of the free end of the vibrational mode.